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Update DeterminateSystems/magic-nix-cache-action action to v15 #242

Job	Run time
Build scip-java CLI	4m 41s
gradle-java21-readiness	2m 35s
maven-java25-demo	2m 29s
gradle-mixed-kotlinpoet	3m 36s
bazel-java-examples	2m 25s
gradle-kotlin-kotlinpoet	3m 53s
gradle-kotlin-kotlinconf-app	5m 26s
maven-kotlin-json-kotlin-maven	2m 51s
maven-java17-spring-boot	2m 52s
Repository indexing pass	2s
	30m 50s